Mustard leaf kimchi contains numerous functional compounds that have various health benefits. However, the underlying mechanisms of their anti-inflammatory effects are unclear. In this study, changes in the mustard leaf kimchi phenolics profile after fermentation with or without Lactobacillus plantarum were determined using liquid chromatography–mass spectrometry/mass spectrometry (LC–MS/MS). To correlate changes in phenolic profiles with anti-inflammatory activities of the fermentation extracts, lipopolysaccharides (LPS)-stimulated RAW 264.7 cells were treated with the extracts. We identified 12 phenolic acids in mustard leaf kimchi fermented with L. plantarum. Caffeic acid, chlorogenic acid, epicatechin, and catechin substituted the metabolite abundance. Extracts of mustard leaf kimchi fermented by L. plantarum (MLKL) markedly inhibited nitric oxide production by decreasing interleukin 6 (IL-6), tumor necrosis factor-α (TNF-α), inducible nitric oxide synthase (iNOS), and cyclooxygenase 2 (COX2) expression levels in LPS-treated RAW 264.7 cells. Thus, fermentation with L. plantarum potentially improves the anti-inflammatory activities of mustard leaf and mustard leaf fermented with this microorganism may serve as a proper diet for the treatment of inflammation.